The present invention relates to methods and means by which thin single-crystal silicon films may be formed on insulating substrates. More particularly, it relates to a method by which thin single-crystal silicon films may be formed on insulating substrates from polycrystalline deposits of silicon on the substrates.
There is a need, at the present time, for silicon films of single crystal variety on insulating substrates for use in the manufacture of very large scale integrated circuits (VLSI). These circuits provide the advantage of achieving high packing densities and speeds in the circuitry.
Currently, a large amount of effort has been concentrated on recrystallizing thin films of polycrystalline silicon by moving concentrated heat sources across a wafer to melt the polycrystalline material as the heated zone advances and to permit recrystallization of silicon at the trailing edge of the molten zone. The several processes by which the heat is generated and applied characterizes the several recrystallization process. Heat sources such as graphite, heaters, strip heaters, lasers, electron beams, quartz halogen lamps, and RF susceptors have all been used as heat sources.
Such efforts are described in a number of publications as follows:
1. M. W. Geis, H. I. Smith, B. Y. Tsaur, J. C. Fan, D. J. Silversmith and R. W. Mountain, J. Electrochem. Soc. 129, 2817 (1982).
2. J. F. Gibbons, Laser and Electron Beam Solid Interactions and Material Processing, ed. J. F. Gibbons, L. D. Hess and T. W. Sigmon (North Holland, N.Y., 1981) p.449.
3. T. I. Kammis and A. C. Greenwalt, Appl. Phys. Lett. 35, 282 (1979).
4. T. J. Stultz and J. F. Gibbons, Appl. Phys. Lett. 41, 824 (1982).
5. Y. Kobayashi, A. Fukami and T. Suzuki, IEEE Electron. Deve. Lett. EDL-4, 132 (1983).
A survey of efforts in these techniques is presented in an RCA Review, Vol. 44 at page 250 written in 1983 by L. Jastrzebski.
Many of the prior art silicon-on-insulator structures have utilized a silicon wafer as a crystalline substrate, overlaid with a layer of silicon dioxide as the insulating substrate. Such substrates have the problem that at temperatures above 1000.degree. C. silicon is mechanically weak and has a yield stress less than that of annealed copper. As a consequence, it is not feasible to pass a hot thermal zone over a silicon wafer without causing plastic yielding and wafer warpage except under those special conditions where the temperature difference from one part of the wafer to another never exceeds 10.degree. C. and therefore where the temperature of the entire wafer must be raised to within 10 degrees of the melting point (1410.degree. C.) of silicon.
Although impressive results have been attained with the thermal zone-melting method, an improved process of recrystallizing thin films of silicon on an insulating base is desirable. For such a process, it is recognized that it would be desirable to use conventional semiconducting processing equipment such as is widely and inexpensively available. Also, it would be desirable that such a process be carried out under isothermal conditions at temperatures far below the melting point of silicon when the wafer is a silicon oxide insulating coating on a single crystal silicon base. Such lower temperature production is feasible by the method of the present invention based on use of eutectic compositions which melt at temperatures substantially below the melting point of silicon.
Also desirably such a process should be a batch oriented process capable of producing 400-500 wafers per run and should result in a low impurity level in the recrystallized silicon. The process of the present invention has this capability.
It has now been discovered that it is possible to produce silicon on insulated wafers which meet all of these criteria and to do so at an economical cost. The method of the present invention is distinct from the zone melting process referred to above in that it does not employ a heat source to impart heat or a temperature gradient to the polycrystalline silicon layer. Nevertheless, it does result in the generation of a zone melt which advances across the polycrystalline layer to melt the polycrystalline silicon as it advances and to leave a single crystal of silicon at the trailing edge of the molten zone. The process is carried out at temperatures far below the melting point of silicon.
Uniquely, the present method harnesses the free energy difference between polycrystalline and single crystal silicon to generate and to drive the molten eutectic zone. For this reason it is referred to herein as a grain-driven zone melting.